Synthetic zeolites are crystalline aluminosilicates, and have uniform fine pores of a size of angstroms which are produced due to the crystalline structure. Using this characteristic, synthetic zeolites are industrially used as a molecular sieve adsorbent which adsorbs only molecules having a specific size, an adsorption separating agent which adsorbs molecules having a strong affinity, or a basis of catalysts.
A variety of zeolite production methods have been proposed. When, for example, beta zeolite, which is one synthetic zeolite, is considered as an example, an ordinary zeolite production method is a method in which tetraethylammonium ions are used as an organic structure-directing agent (hereinafter referred to as an “organic SDA”). Such a method is described in, for example, the following PTL 1. However, while compounds including tetraethylammonium ions are expensive, the compounds are almost decomposed after crystallization of beta zeolite is completed, and therefore it is impossible to collect and reuse the compounds. As a result, beta zeolite produced using this method is expensive. Furthermore, since tetraethylammonium ions are entrapped in the crystals, it is necessary to fire and remove tetraethylammonium ions when beta zeolite is used as an adsorbent or a catalyst. At this time, exhaust gas causes environmental contamination, and a large amount of chemicals is also required for a detoxifying treatment of the mother fluid of synthesis. As such, since a method of synthesizing a zeolite in which the organic SDA is used is a production method which is not only expensive but also causes a large environmental load, there has been a demand for realization of a production method in which the organic SDA is not used.
Under such circumstances, in recent years, a method of synthesizing beta zeolite in which the organic SDA is not used has been proposed (refer to NPL 1). In this method, a substance obtained by firing beta zeolite, which has been synthesized using tetraethylammonium ions, so as to remove organic components is used as seed crystals, the substance is added to a sodium aluminosilicate reaction mixture which does not include any organic substance, and a hydrothermal treatment is carried out, thereby crystallizing beta zeolite. However, in this method, since beta zeolite, which has been synthesized using tetraethylammonium ions, is fired and used as seed crystals, tetraethylammonium ions become necessary at all times as the organic SDA while the amount of the organic SDA used decreases. In addition, according to this method, there is only one kind of seed crystal, and there is only one numerically limited example for the composition of the sodium aluminosilicate reaction mixture. Therefore, while the composition of the synthesized beta zeolite is not clearly described, the composition is considered to have only the determined values.
Meanwhile, PTL 2 by the authors of NPL 1 discloses the SiO2/Al2O3 ratios of seed crystals, and also describes the composition of the sodium aluminosilicate reaction mixture not as a point composition but as a narrow range away from a point. However, since the contents disclosed by PTL 2 are basically the same technique as the contents of NPL 1, and the composition range of the reaction mixture is narrow, the SiO2/Al2O3 ratio of beta zeolite is limited only to a limited range. In order to meet a variety of needs, zeolites having a wide SiO2/Al2O3 ratio range are desirable. In addition, in order to reduce the environmental load as much as possible, there is a demand for proposing a new zeolite production method in which seed crystals which do not need to be fired are used, and the organic SDA is not used.